The technical field of this invention is the deployment of an occupant restraint in a motor vehicle during a vehicle crash event.
Occupant restraint deployment controls have developed greatly, in sophistication, capability and complexity, and perform admirably at distinguishing deploy from non-deploy events. This is often achieved with the use of a xe2x80x9csmartxe2x80x9d crash sensor, consisting of an accelerometer with very sophisticated signal processing and logic, in the passenger area of a vehicle, supplemented by one or more simpler satellite crash sensors in other locations providing additional information to the process. But the process of improvement is by no means over, and further sophistication of the controls continues.
One of the satellite sensors for such systems is located in a xe2x80x9ccrush zonexe2x80x9d of the vehicle, where it provides an earlier look at the accelerations produced by a crash event. New, more sophisticated controls are making more use of the output of such xe2x80x9ccrush zonexe2x80x9d satellite sensors for use in crash severity measures required for multiple stage restraint deployment. The satellite sensors themselves are therefore becoming xe2x80x9csmartxe2x80x9d, with their own sophisticated processing and logic including time dependent thresholds and other features requiring the use of a crash dependent software xe2x80x9cclockxe2x80x9d that provides a dependable indication of time from the initiation of a possible crash event.
The use of a crash dependent clock with a time dependent threshold is illustrated in the curves of FIG. 5, which shows an acceleration signal 4 derived from a crash sensor accelerometer in a possible crash event as a function of time. Superimposed on the curve is a time dependent threshold to which the acceleration signal is to be compared: if the acceleration signal exceeds the threshold prior to a predetermined desired trigger time DTT, a true crash event is indicated. But this threshold is shown as two separate curves, identical in shape but shifted in time: curve 6 is initiated earlier in time than curve 8. The result of this is that the same measure 4 produces a positive crash indication when compared with later initiated threshold curve 8 but does not when compared with the earlier initiated threshold curve 6. Since the generation and timing of the threshold curve is controlled by the crash dependent clock, the operation of that clock is crucial.
The simplest prior art embodiment of a crash dependent clock of the prior art is a software clock variable set to an initial value such as zero and incremented beginning when the acceleration signal exceeds a first predetermined level, typically about 1.5 to 2 g""s, that is higher than any produced by hard braking. The variable continues to be incremented, once each program loop, but is reset if a predetermined number of loops produce acceleration values than a second predetermined level that may be the same as the first or lower for hysteresis.
More sophisticated crash clocks provide for multiple event scenarios wherein the vehicle hits a curb or pothole just before hitting a tree or light pole. Simple crash dependent clocks may combine the initial, non-crash event and the later, true crash event into one long event, with early initiation of a time dependent threshold preventing this threshold from correctly identifying the true crash event. Crash dependent clock algorithms thus now provide for temporarily holding or counting the variable backward to control the time dependent threshold in a more sophisticated manner for more accuracy in complex scenarios.
But there is an additional problem with the use of crash dependent clocks in a xe2x80x9ccrush zonexe2x80x9d of the vehicle. Such an area is close to the actual point of impact and contains significant structure undergoing deformation: it thus tends to be very noisy, in terms of accelerations; and this makes it difficult to pick out a clean acceleration signal for accurate control of a crash dependent clock.
The crash sensor of this invention overcomes noisy vehicle acceleration signals from areas such as a crush zone to provide a crash dependent clock by using a velocity signal derived from the acceleration signal, rather than the acceleration signal itself, to determine initiation and counting of the clock variable. Crash velocity provides a cleaner signal in the crush zone for crash dependent clock determination.
The sensor of this invention is a satellite crash sensor for a motor vehicle occupant restraint system containing an accelerometer for providing an acceleration signal, means for initiating a clock count at an initial clock value, means for sampling the acceleration signal and deriving a velocity value therefrom and means for accumulating successive ones of the velocity values in an accumulated velocity value and limiting the accumulated velocity value between a minimum limit value and a maximum limit value. It further has means for incrementing the clock count in a first direction from the initial clock value when the accumulated velocity value is not equal to the minimum limit value and in an opposite direction toward the initial clock count when the accumulated velocity value is equal to the minimum limit value. Finally, it has means for using the clock count as a measure of time from initiation of a possible crash event in the generation of a restraint deploy indicating signal in the satellite crash sensor.